1. Field of the Invention
The present invention generally relates to the measurement of fluid properties and, more particularly, to the determination of the velocity of a fluid of interest.
2. Description of the Prior Art
A number of techniques have been devised to measure the velocity of a fluid within a lumen. One approach is the "time of flight" approach, which generally includes determining the time require for a thermal wave to flow from a source heater element to a destination sensor element. By knowing the distance between the heater and sensor, the velocity of the fluid can thus be calculated.
U.S. Pat. No. 4,576,050 to Lambert discloses one such "time of flight approach. Lambert suggests energizing a heater strip with an oscillating heater input signal to emit thermal waves in the fluid. The thermal waves propagate through the fluid at a rate that is dependent on the fluid velocity that flows perpendicular to the heater strip. A thermo-electric detector, spaced from one or both side of the heater, senses the thermal wave and provides a corresponding detector output signal. The velocity of the fluid can be determined, at least to first order, from the time differential between the heater input signal and the detector output signal.
A limitation of the Lambert approach is that the measured time or phase differential between the heater input signal and the detector output signal depends on a number of physical properties of the fluid, including, for example, temperature, pressure, thermal conductivity, and thermal diffusivity. To compensate for some of these parameters, Lambert suggests making a reference measurement with a second sensor that is exposed to still fluid or fluid having a velocity component that is perpendicular to the source of the second sensor. By comparing the output of the second sensor with that of the first sensor, Lambert suggests that a phase difference can be computed that is independent of at least some of the fluid properties listed above. As can be seen, the second sensor of Lambert is not used to measure the fluid velocity, but rather is used to compensate for selected physical properties of the fluid.
Another limitation of the Lambert approach is that several potential error sources in the phase differential measurement are neglected, thereby reducing the accuracy of the measurement. One such error source is the non-zero heater time lag that typically exists between the heater input signal and the elevated temperature response of the heater element (and thus the fluid). The heater time lag is typically dominated by the thermal conductivity, k, of the fluid of interest, at least for microbridge structures as contemplated by a preferred embodiment of the present invention.
Another error source is the non-zero sensor time lag that typically exists between the arrival of the temperature disturbance at the sensor element and the corresponding response of the sensor element. The temperature of the sensor element typically does not react instantaneously to a temperature change in the fluid, primarily due to the sensors non-zero thermal mass.
It would be desirable, therefore, to provide an method and apparatus whereby the fluid velocity can be determined relatively independently of the physical properties of the fluid. It would also be desirable to provide a flow sensor that accounts for the non-zero heater time lag and the non-zero sensor time lag for increased accuracy.